**Update 20150427**: A patch has been released and made available by the WordPress Core Team in version 4.2.1 – Please update immediately.
Yes, you’ve read it right: a critical, unpatched 0-day vulnerability affecting WordPress’ comment mechanisms was disclosed earlier today by Klikki Oy.
If your WordPress site allows users to post comments via the WordPress commenting system, you’re at risk. An attacker could leverage a bug in the way comments are stored in the site’s database to insert malicious scripts on your site, thus potentially allowing them to infect your visitors with malware, inject SEO spam or even insert backdoor in the site’s code if the code runs when in a logged-in administrator browser.
You should definitely disable comments on your site until a patch is made available or leverage a WAF to protect your site and customers.
This vulnerability requires an attacker to send a comment long enough to force the backend MySQL database to truncate what is stored.
As you can see from the above schema, the comments texts are stored in the comment_content column which is a TEXT column, meaning a comment can only contain a maximum of 65535 bytes of data.
A typical exploit would look like the following:
<a href='x onclick=alert(1) AAAAAAAAAAAAAA..(multiplied so our comment contains more than 65k bytes)'>test</a>
Once taken back from the database would look like this:
<p><a href='x onclick=alert(1) AAAAAAA</p>
Some of you might have noticed that the resulting HTML tag isn’t complete, but this isn’t a problem for most modern browsers as most of them will simply patch it up automatically:
What you can do
There’s a few thing you can do to prevent getting hacked before there’s an official patch being released: You can disable comments on your site or leverage a Web Application Firewall to filter good requests from exploit attempts.
Hopefully the WordPress team will release a patch soon.